Conventionally, in a multi-channel integrated light receiving circuit used in wavelength multiplexed optical communication, optical cross talk between channels is suppressed by disposing a light receiving element array on the side face of a planar optical circuit. Furthermore, optical cross talk between channels can suppressed by accommodating light receiving elements of the light receiving element array in a light blocking holder (see, e.g., Japanese Laid-Open Patent publication No. 2005-250178).
As a technique to realize large capacity optical communication, a coherent transmission technique has recently been present. In an optical receiving apparatus used in the coherent transmission technique, signal light and a locally oscillated light interfere with each other in an optical waveguide whereby, the signal light is amplified; and the amplified signal light is received by a light receiving element for signal light. Light branched from the signal light before interference with the locally oscillated light is received by a light receiving element to monitor the signal light level.
A portion of the high output locally oscillated light may be reflected by an end face of the optical waveguide and the light receiving element for signal light, thereby scattering and diverging in the optical waveguide and the housing before being received by the light receiving element for monitoring the signal light level. According to the technique of suppressing the optical cross talk among the channels in the traditional multi-channel integrated light receiving circuit, however, it is not assumed that the locally oscillated light is caused to be input into the optical waveguide and to interfere with the signal light. Therefore, cross talk light generated by the locally oscillated light cannot be prevented from being received by the light receiving element for monitoring the signal light level.